How individual Montagu's Harriers cope with Moreau's Paradox during the Sahelian winter
Résumé
1. Hundreds of millions of Afro-Palaearctic migrants winter in the Sahel, a semi-arid beltsouth of the Sahara desert, where they experience deteriorating ecological conditions duringtheir overwintering stay and have to prepare for spring migration when conditions are worst.This well-known phenomenon was first described by R.E. Moreau and is known ever since asMoreau’s Paradox. However, empirical evidence of the deteriorating seasonal ecological conditionsis limited and little is known on how birds respond.2. Montagu’s Harriers Circus pygargus spend 6 months of the year in their wintering areas inthe Sahel. Within the wintering season, birds move gradually to the south, visiting several distinctsites to which they are site-faithful in consecutive years. At the last wintering site, birdsfind themselves at the southern edge of the Sahelian zone and have no other options than facingdeteriorating conditions.3. We tracked 36 Montagu’s Harriers with GPS trackers to study their habitat use and behaviourduring winter and collected data on the abundance of their main prey, grasshoppers, in Senegal.Since grasshopper abundance was positively related to vegetation greenness (measured as normalizeddifference vegetation index, NDVI), we used NDVI values as a proxy for prey abundance inareas where no field data were collected. Prey abundance (grasshopper counts and vegetationgreenness) at wintering sites of Montagu’s Harriers decreased during the wintering period.4. Montagu’s Harriers responded to decreasing food availability by increasing their flighttime during the second half of the winter. Individuals increased flight time more in areas withstronger declines in NDVI values, suggesting that lower food abundance required moreintense foraging to achieve energy requirements. The apparent consequence was that Montagu’sHarriers departed later in spring when their final wintering site had lower NDVI valuesand presumably lower food abundance and consequently arrived later at their breeding site.5. Our results confirmed the suggestions Moreau made 40 years ago: the late wintering periodmight be a bottleneck during the annual cycle with possible carry-over effects to the breedingseason. Ongoing climate change with less rainfall in the Sahel region paired with increasedhuman pressure on natural and agricultural habitats resulting in degradation and desertificationis likely to make this period more demanding, which may negatively impact populationsof migratory birds using the Sahel.
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